As per Bewildered's remarks, you could lose your funding if you go part-time though if the London job is well paid, I gather this will not bother you.
Your supervisor will have probably applied for funding for the project, assuming the project would be finished in the normal three to four years. Going part-tie may strecth this to between six and seven years depending on your University's guidelines (York per chance? - not important, though you are only two hours from London), so he may be quite unhappy. You need to talk to him first though.
You might be in a position where you have to choose between the job and the PhD if your supervisor is not happy about the potential arrangements. That said, if you do pull it off and you are prepared to juggle a part-time PhD alongside a job (and from what I've seen it can be hard) then genuine works experience alongside your PhD will look good on your CV in the future.
Ian